Advances in magnetic resonance (MR) perfusion imaging have provided clinical researchers with the opportunity to measure quantitative regional increases in cerebral blood flow. The purpose of this study is to acquire the technical experience required to perform MR perfusion imaging studies of the hypercapnic cerebral blood flow response. Cerebral blood flow will be increased by inhalation of carbogen (an air mixture containing 6 percent CO2 and used to callabrate experiments for determining oxygen consumption. To date, twelve subjects (seven males/five females) received the air mixture containing 5% CO2 and nine subjects (five males/four females) received the air mixture containing 6% CO2. Cerebral blood flow increased by 20 ? 8% due to inhaling 5% CO2 and by 36 ? 7% due to inhaling 6% CO2. Within-session reproducibility was tested by comparing the relative increases in cerebral blood flow from the two CO2 intervals within each session. Averaged over all subjects the difference between the relative CBF increases in the first and second CO2 intervals was not statistically significant (P value < 0.05). The standard deviation of the difference was 8% of the mean relative CBF increase. These results served as a basis for the submission of new protocol to study the effect of 6% C02 in patients with Multiple Sclerosis to determine if changes in cerebral blood flow would be similar in patients compared to aged-sex matched controls.